1. Field of the Invention
The present invention relates to an air-fuel ratio control device for an engine that controls an air-fuel ratio of the engine based on at least an output of an air-fuel ratio sensor disposed in the exhaust passage upstream of a three-way catalyst. More specifically, the present invention relates to such an air-fuel control device that is able to detect the deterioration of the three-way catalyst based on at least an output of an air-fuel ratio sensor disposed in the exhaust passage downstream of the three-way catalyst.
2. Description of the Related Art
An air-fuel ratio control device for controlling an air-fuel ratio of an engine by a feedback control based on an output of one air-fuel ratio sensor (O.sub.2 sensor) disposed in an exhaust passage upstream of a catalytic converter is known as a single O.sub.2 sensor system. The single O.sub.2 sensor system is used to control the air-fuel ratio of the engine at a stoichiometric air-fuel ratio to improve the condition of exhaust emissions by utilizing the ability of the three-way catalytic converter to a maximum degree.
Also, to compensate for the individual difference among cylinders or changes due to aging of the upstream O.sub.2 sensor, a double O.sub.2 sensor system using two O.sub.2 sensors has been developed (U.S. Pat. No. 4,739,614). In the double O.sub.2 sensor system, O.sub.2 sensors are disposed upstream and downstream of the catalytic converter in the exhaust passage, and the air-fuel ratio control is carried out based on the output of the downstream O.sub.2 sensor as well as the output of the upstream O.sub.2 sensor.
Nevertheless, even in the double O.sub.2 sensor system, if the catalyst in the catalyst converter deteriorates, the condition of the exhaust emissions such as HC, CO, NO.sub.x deteriorates. Therefore, it is necessary to detect the deterioration of the catalyst accurately.
To detect the deterioration of the catalyst in the catalytic converter, various methods and devices have been proposed.
For example, Japanese Unexamined Patent Publication No. 63-97852 discloses a method for detecting the deterioration of the catalyst based on the interval of reversals of the output of the downstream O.sub.2 sensor (i.e., the period of changes of the output signal of the downstream O sensor from a rich side air-fuel ratio to a lean side air-fuel ratio, or vice versa ) during air-fuel ratio feedback control based on the output of the upstream O.sub.2 sensor.
It is known that the interval of reversals of the output of the downstream O.sub.2 sensor during the air-fuel ratio feedback control becomes shorter when the catalyst in the catalytic converter has deteriorated. The method disclosed in Japanese Unexamined Patent Publication No. 63-97852 utilizes this phenomenon to detect the deterioration of the catalyst by counting the number of reversals of the output of the downstream O.sub.2 sensor over a predetermined time period when the air-fuel ratio of the engine is feedback controlled in accordance with the output of the upstream O.sub.2 sensor under predetermined operating conditions of the engine.
If the number of reversals is larger than a predetermined value (i.e., if the intervals of the output of the downstream O.sub.2 sensor becomes shorter), it is determined that the catalyst has deteriorated.
In the above method, the deterioration of the catalyst is determined by detecting a reduction in an storage effect of the catalyst. That is, the catalyst has an ability to adsorb oxygen in the exhaust gas when the air-fuel ratio is in a rich side compared to the stoichiometric air-fuel ratio (i.e., the air-fuel ratio of the exhaust gas is lower than the stoichiometric air-fuel ratio), and to release the oxygen when the air-fuel ratio is in a lean side compared with the stoichiometric air-fuel ratio (i.e., the air-fuel ratio of the exhaust gas is higher than the stoichiometric air-fuel ratio). This ability, i.e., the O.sub.2 storage effect of the catalyst, becomes lower as deterioration of the catalyst proceeds. In the above method, the reduction in the O.sub.2 storage effect is detected by counting the number of reversals of the output of the downstream O.sub.2 sensor.
The deterioration of the catalyst can be detected accurately by utilizing the O.sub.2 storage effect of the catalyst provided that a period of a cycle of the air-fuel ratio feedback control is relatively short. (In this specification, the term "period of a cycle of the air-fuel ratio feedback control" means a period of oscillation of the air-fuel ratio between a rich side air-fuel ratio and a lean side air-fuel ratio when the air-fuel ratio is feedback controlled, i.e., the period represented by "T" in FIG. 1A. )
However, if the period of the cycle of the air-fuel ratio becomes relatively longer, it is difficult to determine the deterioration of the catalyst accurately by utilizing the O.sub.2 storage effect.
This problem is explained in detail with reference to FIGS. 1A to 1G.
FIG. 1A shows a typical response of the air-fuel ratio of the exhaust gas upstream of the catalytic converter when the air-fuel ratio feedback control is carried out. As shown in FIG. 1A, the air-fuel ratio of the exhaust gas oscillates periodically between a rich side air-fuel ratio and a lean side air-fuel ratio so that the central value of the oscillation coincides with the stoichiometric air-fuel ratio. The period of the cycle of the air-fuel ratio feedback control, which is indicated by T in FIG. 1A, is normally relatively short (for example, approximately 0.5 seconds).
FIG. 1B shows the response curve of the output signal VOM of the upstream O.sub.2 sensor when the air-fuel ratio is oscillating, as shown in FIG. 1A. The output signal VOM also oscillates between a rich side and a lean side, and the interval of the reversal of the output signal VOM is same as the period of the cycle of the air-fuel ratio feedback control (i.e., T in FIG. 1A).
FIGS. 1C and 1D show the response curves of the output signal VOS of the downstream O.sub.2 sensor in this case. FIG. 1C shows the response curve when the catalyst is normal, and FIG. 1D shows the same when the catalyst has deteriorated.
If the catalyst is normal, the catalyst adsorbs surplus oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas is in the lean side compared with the stoichiometric air-fuel ratio, and releases the adsorbed oxygen when the air-fuel ratio of the exhaust gas is in the rich side compared with the stoichiometric air-fuel ratio. Therefore, the air-fuel ratio of the exhaust gas downstream of the catalyst is maintained nearly constant at the mean value of the oscillation of the air-fuel ratio of the exhaust gas upstream of the catalyst (i.e., stoichiometric air-fuel ratio) though the air-fuel ratio of the exhaust gas upstream of the catalyst is oscillating. Accordingly, the output signal VOS of the downstream O.sub.2 sensor reverses at a relatively longer interval as shown in FIG. 1C.
On the other hand, if the catalyst is deteriorated, since the O.sub.2 storage effect of the catalyst also becomes lower, the amount of the oxygen which is adsorbed and released from the catalyst decreases. This causes the output signal VOS to oscillate at a short interval of reversals in the same manner as the output VOM of the upstream O.sub.2 sensor (see FIG. 1D). Therefore, it is possible to detect the deterioration of the catalyst easily by monitoring the interval of reversals of the output signal VOS of the downstream O.sub.2 sensor.
However, if the period of the cycle of the air-fuel ratio feedback control becomes longer as shown in FIG. 1E for some reason, the time period in which the air-fuel ratio of the exhaust gas upstream of the catalyst stays in the rich side or the lean side also becomes longer.
If the upstream air-fuel ratio continues to stay in the lean side after the catalyst has adsorbed the oxygen to the maximum adsorbing capacity, the catalyst does not adsorb the surplus oxygen in the exhaust gas. This causes the air-fuel ratio of the exhaust gas down stream of the catalyst to also be in lean side since the surplus oxygen is no longer adsorbed by the catalyst. Similarly, if the upstream air-fuel ratio continues to stay in the rich side still after the catalyst has released all the adsorbed oxygen, the air-fuel ratio of the exhaust gas turns to the rich side since the oxygen is no longer released from the catalyst.
Therefore, when the period of the cycle of the air-fuel ratio feedback control becomes longer (as shown in FIG. 1E), the air-fuel ratio downstream of the catalyst oscillates in a similar manner as the air-fuel ratio upstream of the catalyst, thereby causing the output VOS of the downstream O.sub.2 sensor to oscillate at relatively short interval of reversals in the similar manner as the output VOM of the upstream O.sub.2 sensor regardless of the deterioration of the catalyst (see FIGS. 1F and 1G). In such cases, if the determination of the deterioration of the catalyst is carried out, a normal catalyst can be erroneously determined as being deteriorated.
There are cases in which the period of the cycle of the air-fuel ratio feedback control becomes longer. For example, when the upstream O.sub.2 sensor has deteriorated, the period of the cycle of the air-fuel ratio feedback control becomes longer since the response of the upstream O.sub.2 sensor becomes lower. U.S. Pat. No. 5,134,847 discloses a device for determining the deterioration of the catalyst that can prevent the above mistake in determination due to the deterioration of the upstream O.sub.2 sensor. The device in U.S. Pat. No. 5,134,847 monitors the condition of the upstream O.sub.2 sensor, and prohibits the determining operation when the upstream O.sub.2 sensor is determined as being deteriorated. The upstream O.sub.2 sensor is determined as being deteriorated when the response of the upstream O.sub.2 sensor becomes lower (e.g. when the period of the cycle of the oscillation of the output of the upstream O.sub.2 sensor becomes longer than a predetermined value).
However, even though the upstream O.sub.2 sensor has not deteriorated, the period of the cycle of the air-fuel ratio feedback control can be longer in some cases. For example, in a transition period of the sudden change in the operating conditions of the engine, such as in a sudden acceleration or a deceleration, the period of the cycle of the air-fuel ratio can be longer even though the response of the upstream O.sub.2 sensor is normal. In such a transition period, the output signal VOM of the upstream O.sub.2 sensor may oscillate in the rich air-fuel ratio side or the lean air-fuel ratio side only, but does not oscillate between the rich air-fuel ratio side and the lean air-fuel ratio side (See FIG. 1H). Therefore the period of the cycle of the air-fuel ratio feedback control becomes longer even though the period of the cycle of the oscillation (T.sub.U in FIG. 1H) of the output signal of the upstream O.sub.2 sensor is still short.
Also, when the engine is operated under conditions in which the velocity of the exhaust gas flow in the exhaust passage becomes low, the period of the cycle of the air-fuel ratio feedback control becomes longer since the time required for exhaust gas to flow over the distance between the engine and the position of the upstream O.sub.2 sensor increases.
Therefore, according to the device in U.S. Pat. No. 5,134,847, a normal catalyst can be determined as being deteriorated under such conditions.